The kinestatic charge detector (KCD) is an x-ray radiation detection apparatus which offers significant advantages over more conventional radiography apparatus. In a KCD system, there is provided an x-ray detection volume and a signal collection volume formed in a closed chamber. In the detection volume, there is generally disposed some type of medium which will interact with x-ray radiation to produce secondary energy. The medium is generally enclosed within a defined space and the collection volume is preferably a multi-element detector of secondary energy located at one boundary of the detection volume. An applied electric field across the detection volume imparts a constant drift velocity to secondary energy particles or charges driving the charges of one sign towards the signal collection volume. Charges of the other sign will drift in a direction away from the collection volume and will not contribute to any output signal.
In the operation of the system, an x-ray beam scans a patient and the x-ray radiation passing through the patient is directed into the detection volume. The KCD is oriented such that the one-dimensional array of collector electrodes spans the fan beam which is transverse to the direction of scan, and the width of the x-ray beam in the scan direction is matched to the height of the detection volume. The x-ray radiation collides with particles in the medium of the detection volume creating a secondary energy. The electric field across the detection volume is produced between a first electrode at one side of the detection volume and the plane of the collection volume (collection electrodes) and the direction of the field is substantially perpendicular to the path of the radiation admitted into the detection volume. The electric field causes charge carriers (secondary energy) between the first electrode and the collection electrode to drift toward the collection electrode at a substantially constant drift velocity. The chamber itself, including the detection and collection volumes, is mechanically coupled to an apparatus which moves the chamber in a direction opposite to the direction of drift of the charges at a constant velocity of a magnitude substantially equal to the magnitude of the drift velocity of the charges. The currents flowing in the plural collection electrodes resulting from charges produced on the collection electrodes by the charge carriers are sensed. The spatial distribution in two dimensions of the radiation admitted into the chamber is determined in response to the amplitude with respect to time of the second current flowing in the respective plural collection electrodes. The spatial distribution of radiation in the transverse direction is determined by the spacing of the collection electrodes. Thus, in a KCD system, two-dimensional information can be obtained using a one-dimensional array of collector electrodes.
Since the motion of the chamber is in a direction opposite to the drift of the charge carriers created in the medium in the detection volume, the x-ray radiation passing through each small area of the patient in the x-ray beam is integrated over the time that it takes for the charge carriers in the detection volume to drift through the space of the volume. This integration, which is required in order to obtain adequate signal levels, was achieved in prior art fan-beam systems using two-dimensional arrays of collector electrodes comprised, by way of example, of 80 to 100 elements in the scan direction and 2000 elements in the transverse direction. The KCD system provides the same information using a one-dimensional detector array and thus avoids the cost and complexity of large two-dimensional detector arrays as in the prior art.
Within the detection volume, a grid separates the space between the first electrode and the collector electrode into a drift region and a collection region. The grid shields the collector electrodes from any induced current caused by the charges in the drift region so that only ions in the collection region are detected by the collection electrodes. The spacing between the grid and collection electrodes is one factor affecting the resolution of the system. The data obtained at the collection electrodes is digitally processed to generate an image. In that sense, KCD is a form of digital radiography.
In the operation of the kinestatic charge detector, the drift velocity of the charge carriers or charge clouds within the chamber of the detector must be known so that the detector can be moved at a velocity equal in magnitude to the drift velocity of the charge carriers. The drift velocity is constant only to the extent that the electric field existing between the two spaced electrodes within the detector is uniform, constant and parallel to the desired direction of drift of the charges. Any distortion in the electric field between the electrodes will cause blurring due to variations in the drift velocity and variations in the path length along the electric field lines of force between the electrodes.
One region within the detector in which the electric field may be distorted is in the space in proximity to a front window of the detector at which the x-ray radiation enters. Another region of concern is the rear wall of the detector. In general, the electric field deep within the detection volume is uniform and normal to the collection electrode. However, near the front window the electric field lines of force are bent toward the window. Distortion of the electric field in the area of the radiation admission window reduces the quantum detection efficiency (QDE) of the detector by creating a "dead space" near the front window. Because the field lines of force in the dead space end on the vessel walls or front window rather than the collection electrode, charges formed in the dead space strike the window and do not contribute to the signal output of the collection electrode. In addition, the lines of force which end on the collection electrode in the area of the window are curved rather than straight and cause charge carriers following them to travel over a longer path and with a lower average speed than they would if the lines of force were straight. Image blurring will result because such charge carriers take a longer time to traverse the detection volume and therefore are not stationary in space as the detector is translated at constant velocity.
One proposed solution to reduction of the "dead space" at the front or window of the detector is to dispose an insulative material on the surface of the window and to mount evenly spaced conductive strips oriented in a direction perpendicular to the collection electrodes and perpendicular to the direction of the incident x-ray beam. The conductive strips are connected to an electrical source so that potentials on these strips can be adjusted to create a field which compensates for the electric field distortions normally present near the window of the detector. Alternatively, a highly resistive material may be substituted for the conducting strips. The potential across the resistive material is maintained such that an electric field is created that compensates for the distortions present in the "dead space". Such arrangements, however, cannot totally compensate for field distortions and are progressively more difficult to implement as the allowed amount of distortion is reduced.